Abrasive grinding wheels of the discoidal type with depressed center or the flat type are known which have an outside diameter comprised substantially between 100 and 230 mm and are used particularly on electric or compressed-air portable grinders, also known as sanders, of the high-speed type (60-100 m/s peripheral speed), to perform dressing and/or cutting operations, and are essentially constituted by an abrasive mix reinforced by reinforcements constituted by one or more fabric nets, by one or two metallic annular elements, commonly known as washers or rings, which delimit the hole for coupling the grinding wheel to the pivot of the grinder, and by an optional label made of paper or other applied material commonly used, which adheres to one of the two faces of the grinding wheel (usually to the convex one).
The abrasive mix is constituted generally by grains of abrasive material (light green, dark green, black silicon carbide; corundums, zirconium-modified, semi-friable, red-brown, white, pink, ruby, ceramic-coated, silanized, monocrystalline corundums; sol-gel or sintered ceramic abrasives or others) having a predefined particle size (normally measured in meshes) which are mixed with resins, for example phenolic resins, liquid resins and/or powdered resins, and possibly modified with epoxy, phenoxy and/or other resins, modified with organic and/or vegetable or synthetic compounds, and other types of polyimide resin etcetera, and with additives and fillers.
The reinforcement nets are normally woven with filaments made of glass fiber, but other types of fiber might also be used, such as carbon, Kevlar or others; nets woven with a height in the order of 1.5 meters are immersed in a solution of liquid resins and solvents, wrung between pairs of rollers and dried in appropriate ovens inside which the resin dries without polymerizing, polymerization being completed in the curing oven together with the curing of the grinding wheel.
The discs of net required to reinforce the grinding wheels are obtained by die-cutting or other cutting methods from the net fabric thus impregnated with resin and dried.
The nets may optionally be pre-glued to a sheet of paper or polymeric material of low thickness and also to the labels.
The annular definition elements that delimit the coupling hole of the grinding wheel are constituted by a plate that is annular or has another shape, such as for example a square or polygonal shape, from the internal hole of which a cylindrical or non-cylindrical hollow tab extends; the plate adheres to one of the two faces of the grinding wheel, while the hollow tab enters the hole of the grinding wheel, delimiting its internal wall.
The labels are made of paper or tin foil or other synthetic material and normally are annular (however, they can have any other shape) and can occupy either the entire face of the grinding wheel or a limited area of the face to which they adhere and bear the identification and informational data of the grinding wheel.
One particular known type of high-strength abrasive grinding wheel is described in detail in patent IT 1,334,480, to which reference is made. In particular, the aim of that patent is to provide grinding wheels in which the strength of the layer or layers that determine the removal capacity of the grinding wheel is increased. The mixtures of abrasive material generally used are in fact generally medium-grain abrasive mixes (36, 30 and 24 mesh), which ensure a good abrasive power thereof and make it possible to reduce the uneven wear of the edge of said grinding wheel, which increases as the size of the grains used increases, and furthermore makes it possible to reduce the formation of tooth-like portions and the projection of splinters during use; the above cited patent proposed to use a first reinforcement net with a narrow mesh having a structuring action, immersed in a layer of fine-grain (46-60 mesh) abrasive mix arranged upstream of the layer or layers of the medium-grain abrasive mix provided in the region of contact/work of said grinding wheel. By using this refinement, described in patent IT 1,334,480, it has been observed that it is possible to further increase the particle size of the layer of abrasive material assigned to work (20, 18, 16 and even 12 mesh), with a consequent increase in the abrasive action and therefore in the removal efficiency of said grinding wheel.
However, these known types of grinding wheel are not free from drawbacks, which include the fact that the layer of abrasive mix having high removal capacity, i.e., the medium-grain mix, has very high costs with respect to fine-grain mixes, said cost affecting significantly the total cost of the grinding wheel.
This occurs especially if one uses high-performance mixes, such as mixes based on zirconium-modified corundum.
Furthermore, another drawback is caused by the fact that much of said layer of abrasive mix with high removal capacity, despite being used in providing the grinding wheel, is not used, since it is present in regions of said grinding wheel that are not functionally active.
Furthermore, an additional drawback of known types of abrasive grinding wheel consists in that said grinding wheels having a relatively large thickness with respect to their diameter have a structure that is absolutely rigid and lacks any flexibility, a fact which, in combination with the high rotation rate and imperfections in shape and balancing, causes vibrations at frequencies that are unpleasant and harmful to the upper limbs of the operator.